(a) Field of the Invention
The present invention relates to an apparatus for performing a solution growth of Group II-VI compound semiconductor crystals.
(b) Description of the Prior Art
Such Group II-VI compound semiconductor crystals as Zns amd CdS are the so-called direct gap semiconductors wherein the transition of carriers is of the direct transition type, and the energy band gap of these crystals is generally large.
Accordingly, Group II-VI compound semiconductor crystals are interesting materials for the manufacture of such semiconductor devices as LEDs having unique properties to exhibit spectra in the short wavelength region of blue and violet colors, which cannot be obtained from Group III-V compound semiconductor crystals.
These Group II-VI compound semiconductor source crystals, however, are difficult to accomplish a crystal growth therefrom, and to control their conductivity type when compared with the growth process of Group III-V compound semiconductor crystals. For these reasons, it is the present state of the art that the abovesaid interesting properties of these crystals are not sufficiently made use of effectively.
The energy band gap and the conductivity type of those Group II-VI compound semiconductor crystals which are obtained according to the conventional growth process are shown in Table 1.
TABLE 1 ______________________________________ Physical Crystals Property ZnS ZnSe ZnTe CdS CdSe CdTe ______________________________________ Energy band 3.6 2.67 2.2 2.5 1.74 1.5 gap (eV) Conductivity n type n n p n n p ______________________________________
It should be noted here that the conductivity types obtained as shown in the above table do not represent that they have been controlled as desired, but they are exactly reflecting the result of the crystal growths conducted. Accordingly, it is the present state of art that the carrier concentration of these crystals is not controlled either.
Group III-V compound semiconductor crystals of which one of the crystal-constituent elements has a higher vapor pressure over the other allow an epitaxial growth of a crystal from liquid phase at a relatively low temperature, leading to the development of various kinds of semiconductor devices. On the other hand, Group II-VI compound semiconductor crystals have not shown a significant development in the art with respect to their growth conducted by relying on the abovementioned method, for the following reasons. That is, in any group II-VI compound semiconductor crystal, the respective crystal-constituent elements have high vapor pressures, and also the melting points of these elements are high.
Therefore, in spite of the fact that, as compared with the growth of Group III-V compound semiconductor crystals, the control of the growth conditions for Group II-VI compound semiconductors requires to be more strict, the conventional growth of these latter crystals, in the past, has relied only on such "melt growth" process that is conducted under a high temperature and a high pressure as represented by Bridgman method.
Also, the other reasons why the solution growth technique for the growth of Group III-V compound semiconductor crystals (in practice, it is an epitaxial growth from liquid phase) has not been adopted for the growth of Group II-VI compound semiconductor crystals include, in addition to the fact that the above-mentioned respective crystal-constituent elements have high vapor pressures, the fact that the respective crystals have very low solubility in the "melt" or "solution" of either one of the elements which constitute the crystals.
Taking up, for example, ZnSe, it should be noted that, in the growth temperature zone which permits a solution growth of a ZnSe crystal, the solubility of ZnSe crystal to either the Zn melt or Se melt is extremely poor, leading to a difficulty in performing a solution growth. As a means to obviate this problem, there has been proposed a solution growth method which utilizes the fact that a ZnSe crystal has a solubility, to a certain degree, to Te which is also a Group VI element, and which, based on this fact uses a Te solution to serve as the solvent. In such case, however, the Group II-VI crystal thus obtained is just a mixed crystal, as a natural result, having the composition ZnSe.sub.1-x Te.sub.x' and it has not been possible to obtain a highly pure ZnSe crystal having good crystal perfection.
As a solution growth technique which eliminates many of the above-mentioned drawbacks of the prior art, the present inventor has proposed and disclosed earlier in Japanese Patent Preliminary Publication Nos. Sho 57-77098 and Sho 57-183400 a Group II-VI crystal growth method and an apparatus therefor, based on the invention by the present inventor of "temperature difference technique under controlled vapor pressure", which uses, as the solvent, a solution of one of the crystal-constituent elements having a higher vapor pressure over the other constituent element, and disposes a source crystal in the higher temperature zone of the solution, causing recrystallization to take place in the lower temperature zone of the solution, and further applies to the solution, from thereabove, a vapor pressure of the constituent element having a lower vapor pressure.
However, even when such new method as mentioned above is followed, although the techniques of controlling the crystallinity, the conductivity type and the carrier concentration have made a marked progress, still there have been present the drawbacks in said new method such that the reproduceability of the solution growth process is poor, and that a large-size crystal having good crystal perfection is not obtained.